This invention relates generally to epitaxial growth techniques and more particularly to growth of Group II-VI semiconductor crystalline materials.
As is known in the art, Group II-VI semiconductor epitaxial materials such as cadmium telluride and mercury cadmium telluride have important applications as photodetector elements for detection of electromagnetic energy in the spectral range from approximately 0.8 microns to 30 microns. That is, by adjusting an alloy composition of cadmium and mercury, photodetector elements may be provided which are sensitive to different wavelengths within the 0.8 micrometers to 30 micrometer wavelength band.
Several techniques have been suggested for providing cadmium telluride and mercury cadmium telluride suitable for use in photodector applications. One technique is so-called metalorganic photolytic decomposition as described in an article entitled "Free Radical Mechanism for Photo-Epitaxy" by Irvine et al., Journal of Crystal Growth 79(1986), pp. 371-377, in which metalorganic reactants are introduced into a reactor vessel and the metalorganics are illuminated with high powered radiation to photolytically break bonds in the metalorganics and deposit the material. This technique is generally referred to as a photolytic technique since the initial reactions involved in the process (i.e. the reaction which cracks the metalorganics) are photolytically driven. The subsequent reactions are pyrolytic in which the free radical produced by cracking the metalorganic is combined with free hydrogen and evolved. Several problems are associated with this technique. The region of epitaxial growth on the substrate occurs within a very narrow area defined by the focused width of the illuminating source which is quite small compared to practical substrate diameters. Outside the epitaxial growth region, a homogeneous nucleation layer is provided which is unsuitable for semiconductors applications. Considering the growth region observed, reported growth rates of 0.4 .mu.m/h to 0.8 .mu.m/h are relatively low. The low growth rates would negate some of the advantages achieved by low temperature decomposition using the photolytic technique. In papers entitled "Growth of High Mobility N-Type CdTe by Photo-Assisted Molecular Beam Epitaxy" by Bicknell et al., Applied Physic Letters 49(17), October 1986, pp. 1095-1097 and P-Type CdTe Epilayers Grown by Photo-assisted Molecular Beam Epitaxy, Bicknell et al., Applied Physic Letters 49(25), December 1986, pp. 1735-1737, the authors describe a process of dopant activation on a CdTe grown layer by using an argon laser to activate dopant species at the surface of a CdTe substrate during molecular beam epitaxial growth.
A second technique suggested is metalorganic vapor phase epitaxy (MOVPE), also referred to as metalorganic chemical vapor deposition (MOCVD). As it is known, the MOCVD technique for growing mercury cadmium telluride involves directing vapors comprising mercury, dimethylcadmium, and diethyltelluride into a reactor vessel and chemically reacting directed vapors to epitaxially deposit the Group II-VI material. This technique in which a primary alkyl of the Group VI element here tellurium is directed into the reactor vessel allows for growth of Group II-VI materials at temperatures above about 400.degree. C.
It is also known in the art, as set forth in a paper entitled "Low Temperature Metalorganic Growth of CdTe and HgCdTe Films using Ditertiarybutyltelluride" by W. E. Hoke et al, Applied Physics Letters 48, No. 24, June 16, 1986 and as set forth in a paper entitled "Metalorganic Growth of CdTe and HgCdTe Epitaxial Films at a Reduced Substrate Temperature using Diisopropyltelluride" by W. E. Hoke et al, Applied Physics Letters 46, No. 4, Feb. 15, 1985, that MOCVD growth of Group II-VI materials such as mercury cadmium telluride is possible at temperatures lower than 400.degree. C. Low temperature growth of Group II-VI materials is very desirable for several reasons. Low temperature growth enables the minimization of homojunction or heterojunction interdiffusion, reduces the mercury vacancy concentration, reduces foreign substrate out diffusion into the epitaxial layers, and permits the growth of ultra thin epitaxial structures. In addition to the advantages of low temperature MOCVD growth, one particular advantage with this technique is that relatively high growth rates typically 1.6 .mu.m/h at 230.degree. C. and 25.5 .mu.m/h at 320.degree. C. are provided over practical sized substrate areas.
We have found, however, that low temperature growth provides a separate set of problems which are not generally present in films grown by the MOCVD technique at temperatures of about 400.degree. C. and above. With CdTe films grown at temperatures below 400.degree. C. particularly those grown at temperatures less than about 360.degree. C., the crystalline quality (i.e. the defect state concentration) of the epitaxial films degrades significantly from theoretical values with decreasing growth temperature. Consequently, the transport properties of epitaxial HgCdTe films will also appear to degrade with increasing cadmium telluride mole fraction and reduced growth temperature. For example, with Hg.sub.1-x Cd.sub.x Te material where x is equal to 0.3 indicating a higher concentration of cadmium telluride, the actual charge transport is expected to be lower than the theoretical charge transport for x=0.3 material.
Accordingly, it is desired to provide techniques of metalorganic chemical vapor deposition of Group II-VI materials at low temperatures to maintain the benefits of low temperature metalorganic chemical vapor deposition while reducing the problems encountered with respect to the crystalline quality.